1. Technical Field
The present invention relates to a printed wiring board manufacturing method, and in particular to a printed wiring board manufacturing method whereby a circuit pattern is formed on both sides of an insulating material.
2. Background of the Art
<<Technological Background>>
A printed wiring board, for example, a flexible printed wiring board provided with flexibility and bendability has made remarkable progress in precision, function, thickness and weight. In particular, high density and miniaturization of a formed circuit pattern is outstanding.
A large number of minute through-holes is made in a printed wiring board for connecting a circuit pattern on both sides and for mounting semiconductor parts and the like. Two major methods for making such a through-hole during the process for manufacturing the printed wiring board are a panel plating method and a button plating method.
FIG. 4 is an explanatory front cross-sectional view showing an enlarged substantial part provided to explain a conventional printed wiring board manufacturing method of this kind. FIG. 4A shows a printed wiring board manufactured according to a panel plating method. FIG. 4B shows a coating process according to a button plating method, FIG. 4C shows an exposure process according to the button plating method and FIG. 4D shows a printed wiring board manufactured by the button plating method.
<<Panel Plating Method>>
Referring to a panel plating method as shown in FIG. 4A, a copper foil 2 is attached to both sides (i.e., front side and rear side) of an insulating material 1 to form a substrate 3. After a through-hole 5 for a through-hole 4 is made in the substrate 3, the inner wall surface and the like of the through-hole 5 are made electrically conductive. Then, copper electroplating 6 is applied on the entire substrate 3. In this case, the copper electroplating 6 is not only applied on the inner wall surface of the through-hole 5 of the substrate 3 which becomes electrically conducting, but also on the copper foil 2 attached to both sides.
A through-hole 5 of the substrate 3 is made electrically conducting by such a copper electroplating 6 to obtain electrical conduction on both sides. A circuit pattern 7 is thus formed by going through known steps such as exposure, development, etching and stripping in that order. In this manner, a printed wiring board A as shown in FIG. 4A, for example, a flexible printed wiring board is manufactured.
<<Button Plating Method>>
In the case of a button plating method shown in FIGS. 4B, 4C and 4D, a copper foil 2 is attached to both sides (front side and rear side) of an insulating material 1 to form a substrate 3 in the same manner as the panel plating method described above. A through-hole 5 for a through-hole 4 is made in the substrate 3 before the inner wall surface of the through-hole 5 becomes electrically conducting.
As shown in FIG. 4B, both sides of the substrate 3 are first coated with a photosensitive dry film 8 having a separator attached thereto. As shown in FIG. 4C, a photo mask 9, which is a negative mask, is positioned on the outside of the photosensitive dry film 8 to be exposed and developed by a developing solution.
The photosensitive dry film 8 remains hard except for the vicinity of an opening section of the through-hole 5 of the substrate 3. Copper electroplating 6 is applied on such a hardened photosensitive dry film 8 making it a plating resist. In other words, the copper electroplating 6 has been selectively implemented only on the electrically conducting inner wall surface of the through-hole 5 and the periphery of the opening section of the through-hole 5 on both sides. In this manner, the copper electroplating 6 is formed on the inner wall surface of the through-hole 5 (through-hole 4) and the periphery of the opening section of the through-hole 5 (through-hole 4) on both sides. In this case, the copper electroplating 6 is formed as a substantially button shaped section C.
In this manner, the through-hole 5 becomes electrically conducting to obtain electrical conduction on both sides. The substrate 3 is formed with a circuit pattern 7 by going through the known steps. As a result, as shown in FIG. 4D, a printed wiring board B, for example, a flexible printed wiring board, is manufactured.
<<Prior-Art Document Information>>
A conventional printed wiring board manufacturing method according to such a button plating method is disclosed in the following Patent Document 1.
Patent Document 1: Japanese Unexamined Patent Publication No. 195849/1999
3. Problems to be Solved by the Invention
The following problems have been pointed out in such a conventional printed wiring board manufacturing method.
<<First Problem>>
In a method for manufacturing a printed wiring board A according to a panel plating method, it is pointed that the manufactured printed wiring board A, in particular, a flexible printed wiring board, has a problem with flexibility and bendability.
In the panel plating method, as described above, a copper electroplating 6 is applied not only on a through-hole 5, but also on a copper foil 2 on both sides (front side and rear side) of a substrate 3. In other words, in this substrate 3, copper has been separated out even from the outer surface of a copper foil 2 on both sides for forming a circuit pattern 7 for electrical conduction of the through-hole 5 (the through-hole 4).
Referring to the printed wiring board A manufactured in this manner, since the copper electroplating 6 is thoroughly applied on the outer surface of all the circuit patterns 7 formed on both sides, flexibility and bendability has deteriorated accordingly.
The printed wiring board A manufactured by the panel plating method has a problem with flexibility in that, for example, a bendable wiring section 10 as shown in a schematic plan view of FIG. 5 cannot be readily folded or bent in use. This problem is particularly remarkable in the flexible printed wiring board.
<<Second Problem>>
In the manufacturing method of the printed wiring board A according to the panel plating method, it is pointed out that the printed wiring board A has a problem with weight and thickness. It is also pointed out that there is a problem in high density and miniaturization of the circuit pattern 7.
Namely, in the printed wiring board A manufactured by the panel plating method, as described in the first problem, the copper electroplating 6 is thoroughly applied even on the outer surface of all the circuit patterns 7 formed on both sides (front side and rear side). It is therefore pointed out that the printed wiring board A has more weight and thickness and goes against the progress of lightness in weight and reduction in thickness.
Referring to the formation of the circuit pattern 7 on both sides in a final process, it is pointed out that the copper electroplating 6 formed on the outer surface of the copper foil 2 of the substrate 3 works against etching and it is difficult to form the high-density, miniaturized circuit pattern 7.
<<Third Problem>>
In the manufacturing method of a printed wiring board B according to a button plating method, it is pointed out that there is a problem in that the manufacturing process is long and complicated, and the process yield is poor and, as a result, affects the cost.
As described above, in the button plating method, since an exposure process and a developing process using the photosensitive dry film 8 and a photo mask 9 for the plating resist are needed prior to the copper electroplating of the substrate 3, this makes the manufacturing process complicated and troublesome.
Further, a location of the through-hole 5 on the substrate 3 side and a location of the photo mask 9 side corresponding to the through-hole 5 are visually positioned, but this positioning operation is not easy but very troublesome.
Thus, the button plating method not only improves the drawbacks of the panel plating method described above, but also excels in flexibility and bendability because the copper electroplating 6 is applied only in the vicinity of the through-hole 4. Further, the button plating method has no problem with weight and thickness and can respond to the high density and miniaturization as well. However, it is pointed out that this button plating method has a complicated and troublesome process, of which the yield is poor, and has a big problem with cost.
<<Fourth Problem>>
In the manufacturing method of a printed wiring board B according to the button plating method, it is pointed out that the manufacture printed wiring board B also has a problem with the positioning accuracy and the like of the copper electroplating 6 on the through-hole 4.
Namely, in the case of the button plating method, as described in the third problem above, positioning of the through-hole 5 of the substrate 3 and the photo mask 9 is conducted, but displacement therebetween often occurs. In other words, since this positioning is often visually conducted, it is difficult to ensure correct positioning because there is possible expansion and contraction in the substrate 3 and the photo mask 9.
Referring to the printed wiring board B manufactured by the button plating method, it is pointed out that there is a drawback in the positioning accuracy of the copper electroplating 6 in the vicinity of the through-hole 5 and the printed wiring board B does not exhibit the rapid progress in high precision.
Further, as shown in an explanatory plan view of FIG. 3B, it is difficult to miniaturize the copper electroplating 6, that is, a button section C, formed in a button shape on the periphery of an opening section of the through-hole 4 due to the problem with such positioning accuracy. As a result, the button section C cannot help having a larger diameter of about 0.2 mm. In this manner, since the button section C has a larger diameter, the diameter of a land 13 formed in response to the circuit pattern 7 also becomes large.
Referring further to the printed wiring board B manufactured by the button plating method, as shown in an explanatory plan view of FIG. 3D, the amount of wiring of the circuit pattern 7 to be put between the lands 13 is small and the wiring density also tends to be limited to a lesser amount. Accordingly, it is pointed out that the printed wiring board B does not exhibit the rapid progress in high function.